Control circuits for static inverters



Aug. 20, 1963 M. LILIENSTEIN ETAL CONTROL CIRCUITS FOR STATIC INVERTERS I 2 Sheets-Sheet 1 Filed 00T.. 20, 1961.

Aug. 20, 1963 M. LILIENSTEIN ETAL.

CONTROL CIRCUITS FOR STATIC INVERTERS Filed 00T'. 20, 1961 2 Sheets-Sheet 2 www@ United States Patent O CONTROL CHQCUITS FR STATIC INVERTERS Manfred Liiienstein, Rolling Hills, and Daniel.

Schaefer, Long Beach, Calif., assignors to American Electronics, inc., Fullerton, Calif., a corporation of California Filed Oct. 20, 1961, Ser. No. 146,654 6 Claims. (Cl. 321-45) The present invention relates to improved control circuits -for particular use in static inverters which change direct current into alternating current, but which have general application; @and the invention relates more particularly to improved control circuits for voltage regulation and protective purposes in static inverters and in kother electronic and electrical systems. l

Copending applicationSerial No.92,706, 'led March l, 1961, in the name of Manfred Lilienstein discloses and claims an improved inverter system for changing direct current power into single-phase or multi-phase alternating current power and for achieving this conversion without the need Ifor mechanical moving parts. The system disclosed in the copending application is a three-phase system, with control circuits and means being provided in the system for voltage regulation of the individual phases and 'for protecting .the components and circui-t-ry `of the system in the event of misiire of the solid state devices included therein. These solid -state devices `are controlled, as fully described in the copending application, so as to produce the alternating current power through a gated switching action.

The circuit and system of the present invention also has utility in a single-phase, or in ia multi-phase static inverter system, of the type described in the copending application. For' the purpose of the present description, `the control circuits of the invention will be described as used in conjunction with ya .single-phase static inverter.` It 'will fbe understood, however, that by duplication 'of the equipment and circuitry to Ibe described, the -system tot ythe invention can similarly be used in multi-phase lstatic inverters. It will also be evident as the description proceeds, that the voltage regulation and protective control circuits of the invention have general utility in other types of systems.

It is often important in single-phase systems lfor the amplitude of the output voltage to be maintained constant, and it is most important in multi-phase systems that the amplitude of the different phases be precisely regulated so that equal amplitudes of all the phases maybe maintained.

An important object of the present invention is -to pro vide an improved voltage regulatingcontrol circuit which may be used in a single-phase static inverter :for precise regulation of the output voltage, and which may be used inf each of the phase channels of a multi-phase static inverterfor maintaining precise `symmetry in the .amplitudes of the different phases of the output Voltage.

' Another object of the invention is to provide such an improved voltage regulating circuit and lsystem which has general utility wherever precise `amplitude regulation of an output signal is required.

Another object of the invention is to providesuch `an improved regulating circuit which is simple in its construction, and which utilizes rugged components so as to be capable of handling high power levels to be used in relativelyhigh power systems.

A more particular object is to provide such .an improved regulating circuit and system which carries out its voltage regulating function without the need for mechanically moving parts, and which utilizes solid state switching devices in the embodiment to be described -to achieve its rugged characteristic and high power capability.

A problem that has been encountered in the past in static inverter systems using solid state switching devices is the tendency under some conditions for the devices to Inisre and thereby cause damage to the circuiu'y and associated components. This misire condition in effect places a direct short circuit 'on the system and, unless some protective measures are taken, damage will occur.

Another object of the invention, accordingly, is to provide an improved control circuit for use in la static inverter system which is capable of responding to a misne condition in the `system eiectively to cle-energize the system and prevent damage thereto.

Another object is to provide such an improved protective circuit which has general utility 'and which is capable of responding to rover-load conditions to provide a control or indicating signal.

A more particular object is to provide such an improved protective circuit, which includes solid state switching devices in the embodiment to be described, and which performs its protective purposes without the need for rnechanical movinglparts.

'Other objects and advantages or the invention will \be come evident from `a consideration :of the following description when taken in conjunction with the accompanying drawings, in which:

FIGURE l is a block diagram of an inventer system tor changing direct current into alternating current `and which may incorporate the control circuits tot the present invention;

FIGURE 2 is a circuit diagram of a voltage regulator circuit constructed in accordance with ione embodiment of the invention and which may be incorporated into the system of FIGURE 1; and

FIGURE 3 is `a misre protection circuit constructed in accordance with a further embodiment of the invention and which also may be included in Ithe system ci FIG- URE 1.

The inverter Isystem of FIGUR-E 1 includes a pulse generator 10. The pulse ygenerator may, tor example, take the form of `an :appropriate transistorized oscillator circuit which includes a source of direct current potential. The source yof direct current potential may, lfor example, take the form of a ibattery. The direct current potential from the source is converted by the pulse generator 10 into into =a series of output pulses:

The output pulses from the pulse generator 10 may have any desired repetition rate. The frequency yot these output pulses determines the frequency of the alternating current output derived from the system of FIGURE 1.

The pulse generator 10 is coupled to ia dip-flop 12, and the Ioutput pulses from the pulse generator trigger the flip-flop between its two stable states. Flip-.flop circuits, per se, are well known to the art, and .any appropriate circuit may be used. The flip-flop 12 produces a rectangular routput wave as it i-s triggered between its two stable states. The rectangular output wave from the flip-dop 12 is applied to a solid state switching circuit and voltage regulator unit 14. As will be Adescribed in conjunction with FIGURE 2, the rectangular wave from the dip-flop is used to control the unit 14 in a manner such that a high power rectangular wave output signal may be derived from the unit.

The unit 14, in turn, is coupled to a transform-er circuit 16. The transformer circuit also will be described in conjunction with FIGURE 2. The output from the transformer circuit is applied to an amplier `and wave Shaper circuit 16. The latter circuit serves to shape the signal from the transformer circuit into, for example, .a sinusoidal signal of a relatively high power level.

The system of FIGURE l includes a misiire protection circuit 18. This circuit, as will be Idescribed in conjunction with FIGURE 3, protects the system from damage due to short circuit currents. These short circuit currents are caused by a misfire condition in the solid state switches included in the unit 14, as will be described.

The system of FIGURE 1 is similar to the corresponding system descnibed in the copending application Serial No. 92,706. For a multi-phase static inverter, a system like the system of FIGURE l is included for each of the phases. For the multi-phase system, however, it is usual to replace the dip-flop 12 with a ring counter, or similar device, so that appropriately phased rectangular wave gating signals may be sequentially applied to the unit 14 in each ofthe @different phases. p

'Ihe solid state switching and voltage regulator system 14, as shown in circuit detail in FIGURE 2, includes a transformer 100' which, in turn, includes a single magnetic core 101. A primary winding 102 is wound on the core 101, and a control winding 104 and a secondary winding 10161are also wound on the core.

The sides of the primary winding 102 are respectively connected to the anodes of a pair of diodes 108, 110. The cathode of the diode 8 is connected to the anode of a controlled solid state device, such as a silicon controlled rectitier 112; and the cathode of the diode 111i is connected to the anode of a controlled solid state device, such as a silicon controlled rectilier 114.

'Ilhe silicon controlled rectiiiers to be described herein, including the silicon controlled rectiiers 112 and 114, may be of any well known type. As is well known, a silicon controlled rectirieris a solid state deuicewhich may be selectively controlled to be rendered conductive or noncondnctive, and which is capable of switching relatively high power levels. The silicon controlled rectifier is considered to be equivalent in functional characteristics to the well known Thyratron discharge tubes.

yThe primary winding 102 of the transformer lll/0' also has a center tap 115 which is connected to a terminal B.

"IThe terminal B derives a direct current exciting potential for the silicon controlled rectiriers 112 and 114 through the misiire protection circuit 18, as will be described in Vconjunction with FIGURE 3.

The primary winding 102 also has additional intermediate taps 116 and 118, these taps being positioned Vsymmetrically on opposite sides of the center tap 115i. f The intermediate tap 116 is connected to the anode of a silicon controlled rectifier 120', and the intermediate tap 118 is connected to the anode of a silicon controlled rectifier 122. The cathode of the silicon controlled rectirier 120 is connected to the anode o-f the silicon controlled The cathodes of the diodes are connected to a terminal A, which is connected to the protection circuit 18 of FIGURE 3, as will be described.

The sides of the control winding 104 of the .transformer 100 are respectively connected to the anodes of a pair of diodes 130 and 132. The cathodes of the diodes 130i and 132 are connected through an inductance choke coil 135 to a resistor 134 and to the cathode of a Zener diode 136 in a bridge circuit 138. The center tap of the control Iwinding 1014 is connected to a resistor 140 and to a resistor 142 in the bridge circuit.

The resistor 134 may, for example, have a resistance of 500 ohms, and it is connected to one terminal of a potentiometer 144. The resistor 140 may, for example also have a resistance of 500 ohms, and the latter resistor is connected to the other terminal of the potentiometer. The potentiometer 144 has a movable arm which is connected to one terminal of the control winding 145 of a magnetic amplier 146.

The resistor 142 may, for example, have a resistance of ings is connected to the above-mentioned terminal B.-

The other terminal of the output winding 148 is connected through a resistor 152 to the anode of a diode 1,54, and the other terminal of the winding 154i is connected through a resistor 156 to the anode of a diode 158. The resistors 152 and 156` may each have a resistance of 180 ohms.

The cathode of the diode 154 lis connected to the gate electrode of the silicon controlled rectiier 1201. A diode 160 has its anode connected to the cathode and its cathode connected to the lgate electrode of the silicon controlled rectifier 121i. A resistor 162 is shunted across the diode 160, las is an inductance coil 164. The resistor 162 may, for example, have a resistance of 47 ohms. A diode 1156 has its anode connected to .the cathode and its cathode connected to the gate electrode of the silicon controlled rectiiier 122. A resistor 168 is connected across the diode 166, as is an inductance coil 170. The resistor 168 may, for example, have a resistance of 47 ohms.

As noted above,` the pulse generator 1d may be of any suitable known type, as may the flip-liep 12. The lfrequency of the pulse generator, as also mentioned, determines the frequency of the output of the system. The flip-flop 12 has a pair of output terminals '-172 and 174. The output terminal 172 is connected through a resistor 176 to the cathode of a Zener diode 178 and to a resistor 180. The resistor 180i is connected to a grounded inductance coil 282. The resistor 176 may, for example, have a resistance of 2010 ohms, and the resistor 181) may, for example, have a resistance of 1010 ohms.

The output terminal 174 is connected through a resistor 182 to the cathode of a Zener idiode 184 and to a resistor 186. The resistor 186 is connected to a grounded inductance coil 138. The resistor 182 may have a resistance of 20() ohms, and the resistor 186 may have a resistance of `1001 ohms. Y

The junction of `the resistors 176 and 178 is connected to the anode of `a diode 190, and the junction of the resistors 182 and 186 is connected to the anode of a diode 192. The cathodes of the 4diodes 19d and 192 are connected to a terminal C. 'Ihe terminal C is connected to f the misfire protection circuit 18 of FIGURE 3, as will be described. l f

The anode of the Zener diode 17 8 is connected to the gate electrode of the silicon controlled rectier 112. A resistor 194 and a diode 196 are connected between the `gate and the cathode of the silicon controlled rectilier 112. The resistor 194 may, for example, have a resistance of 47 ohms.

Ilhe anode yof the Zener diode 184 is connected to the gate `of the silicon controlled rectiii-er 114. A yresistor 198 and a diode 200 are connected between the gate `and the cathode of the silicon controlled rectifier 114. The resistor 198 has a resistance, for example, of 47 ohms.

The circuit of FIGURE 2 yoperates generally in a manner similar to the corresponding circuit in the system ofthe copending application Serial No. 92,706. As mentioned above, a direct current exciting potential is applied to the center tap of the primary Winding 102 of the transformer 100. The silicon controlled rectiiiers` 112 and 114 are alternately switched between conductive and nonconduct-ive states by the action of the dip-flop 12. This switching action causes current to dow in a rst half of the primary winding 102 and in la iirst direction `for a first half-cycle of each operating cycle; and the switching -action causes current to flow in the other half of the primary `1112 and in the opposite direction for a second halfcycle of each operating cycle.

As la result of the switching action described in the Apreceding paragraph, and ofi the' correspon-ding ow of primary currents Iin the primary winding` 102, an alternatin-gcurrent secondary voltage is induced in 'the` secondary winding 106. The alternating current secondary i voltage, as mentioned above, may ,be amplified and shaped to produce a` `sine wave at theoutput of the system. The amplitude of the sine wave may be precisely regulated in a manner nowto be described.

An alternating current..voltage is also induced inthe control Winding 104 by the above-described switching action of the silicon :controlled rectiers 112 and y114. Ilhis `voltage appears across the control winding, and it is related -in amplitude to thesecondary Voltage across the secondary winding 1116. Any variation in the amplit-ude of the alternatinglcurrent secondary voltage across the winding 106 is accompanied by a corresponding variation inthe amplitude of the alternating current control f voltage across `the control winding 4104. The alternating `current control voltage across :the control winding i-s rectilied by the full Wave rect-ifying` diodes 130and 132. 'I'he l i resulting direct `current-control voltage is smoothed by the choke coil 135 `and applied across the bridge 138.

The ydirect current control voltage across the bridge 138 isY applied across the control coil 145 of the magnetic amplifier 146. The. resulting Voltage` across the` output @windings 14S-and15lor". the magnetic `ampliier are Y applied.` ton-the. respective gate electrodes .of the vsilicon controlled rectiiiers 12tl`=and 122. In this manner, the

- time within.` the rst `half-cycle otfeach operating `cycle when its anode voltagerises to a tiring threshold which, in turn, is dependent upon the control voltage introduced toits gate electrode.` Thiscontrol lvoltage is, itself, determined .byithe `amplitude 'of-thevoltage across the control. winding' 104, whichin turn isirel-ated'to the amplitude of the secondary voltagewacrossnhe secondarywind- .ing 106. i f =\Likewise, the silicon controlled rectifier 122 is conf. trolledto fire at a timewithinithe second half cycle of .each 'operatingcyclefwhen `its anode voltage rises to a tiring` threshold determinedbylthe control lvoltage applied to fits .gate electrode.` The gatev voltage of "the, silicon controlled rectifier. 1221s also controlled in accordance with theamplitnde yof the control 'voltage across the A Winding 104.

When thesiliccn controlledrectiiiers 12d) and 122 ,are

. fired, `the current flowthrou-ghLthe silicon controlled recti- `ners filaments news through the` silicon controlled rectiers 120 and 122, rather than through the diodes .198 and `110. Thisiswitching action,therefore, decreases the effective` turns on the primary winding 102, and prov duces Ia corresponding increase inthe secondary voltage n for the latter portion of each of the lhalf cycles. The sec- .,ondaryvoltage across the` secondary winding 5106 is, in-

creased, therefore, ineachthalf4 cycle by the ringotthe i .-`si=licon.controlled Irectiersgj- 2lland y122;and the time at whichfthesev silicon controlledv rectiers are tired in eachlbperatingcycle is, in turn, dependent `rupon the am- .plitude Aof the. secondary voltage.

w:lishcdgby1 the setting ofgthe potentiometer 144. Any

, .-tendency for the secondary ,voltage to vary from tbeset value, is `acccnnpanied by lan earlier or later tiring of the .'silic-onpontrolled rectitiers 129 and 122 -inA each loperating V,cy`cle,dependingupon the Aclirectionof the-variation. 'llhis earlier-or ylaterdiring of-'the silicon (controlled `rectifiers .122 alterstheecondaryfvoltage in ahldrirection t 'llo ,prevent any likelihood ci saturation in' the core 101 ofgthe transformeniit is,V desirable thatthe circuits of ,thesiliecn controlled rectiliers 1Z0-and 122 be balanced,

y.andthatpthe tiring controla/)fI the silicon controlled recti- .e.rS. 120. vani-1.22.1@ symmetrical# Saturation effects in,

. Theccntrol may be; such, therefore, that regulation isl provi-ded for Ithe secondary voltageat anam-plitnde estabthe core 101 of the transformer c'ould arrest the switching actionof the system and Iproduce la loss in output voltage.

The misre protection circuit 18 of FIGURE 3 includes a lead2tl2 which is connected to the` abovementioned terminalB ,of FIGURE 2; and it also includes a lead 204 which is connected to the `above-mentioned terminal A and a lead` 206 which is connected to the above-mentioned terminal C. Thelead 202 is` connected through the main winding 2118 of a magnetic indu-chance device 2101having a core 211, and through an inductancc coil 212 to the positive -terminalofv a source. 214 of direct voltage. The source21-4 is represented as -a battery which may have a direct current potential, for example, ofv

of .the devi-ce 210 produces a ilux in the core 211 which over-rides the. linx produced` by the.V Winding 208 and which drives the core 211 tosaturation in one direction. However, `in the eve-nt yct a misiire in the circuit of FIG- URE 3, the resulting high short circuit current flow through the Winding 24l8, and the resulting flux -in the c-oref211 of .the device 210, drives the core to saturation in the opposite direction.

The magnetic core inductancedevice '210 includes a V.third winding 222 on the core`21-1. When .the magnetic4 flux in the core is reversed due tothe above-described short-circuit condition; therefore, in the presence of the mistire short-circuit current flow in the primary winding of the transformer of .FIGURE 2, a voltage pulse is produced across the Winding 222 of the device 211.

One side of they winding 222 is grounded and the-other is connected to a grounded resistor 224,` The resistor \224,may, for example, have a resistance of l kilo-ohm.

The ungrounded side of the winding 224 is also connected .tothe anode oa diode 226. The cathode of the diode 2261s connectedto a resistor 22S which may, for example, have a resistance of 180 ohms.

The resistor 228 is connected ytothe gate `electrode of Aa silicon controlled rectifier/210, and the resistoris also connected to the cathodeof a diode 212 and. to a grounded resistor 214. The cathode of the silicon controlled rectifier 210, and the anode of the diode 212 are grounded. The resistor 214 may, for example, have a resistance of 47 ohms. 1

Thefanode of the silicon controlled rectier is coupled through .a capacitor 236 to the lead 204 which, as mentioned above, is connected to the aforementioned terminal A of the circuit of FIGURE 2. The capacitor 23tl.may, for example, have acapacitance of microfarads. The anode of theA silicon controlled" rectifier is also drectly connected to a point 232 in .thecircuit of FIGURE 3. The lead 2114 is connected to a resistor 234.which,zin turn, is connected to a grounded resistor 236. The resistor 234 may have a resistance1of,..for example, 25 kilo-ohms, Iand the resistor 236 may have a resistance of 5 kilo-ohms.-

The junction of the resistors 234 and 236 isconnected Y to one side or a shunt network 238. The shunt network includes'a control winding 240 associated'with the. magnetic amplifier 146 of FIGURE 2, anda parallel resistor 242. The resistor may havea resistance, for `example, of 470 ohms. The other side of theshunting network 238 -is connected to alresistor 244. .The resistor 244 may have a resistance of 5 kilo-ohms, for example, and itis connected yto the cathode of a diode 246. The. anode of the` diode 246 is connected `to the lead 202. This latter lead, as mentioned above, is con- 252 and 254, each of the transistors having a grounded emitter. The collectors of the transistors are connected to the opposite sides of the primary winding 256 of a transformer 25?.v l The transformer has a secondary WindingZt). One side of the secondary winding 260 is connected to the cathode of a diode 262 and to a resistor 264. The other side ofthe secondary winding 264D is connected to the cathode of a `diode 266 and to a resistor 268. f

The anodes ofy the diodes 262 and 266 are connected to a grounded resistor 270 which may, forexample, have a resistance of kilo-ohms, and which is shunted by a diode 272.V The resistor 264 may have a resistance, for

example, of 1 kilo-ohm, and it is shunted by a capacitor 274. The` resistor 268, may also have a resistance of l kiio-ohm, `and it is shunted. by `a capacitor 276. The capacitors 274 and276 may each have a capacity, for example, of .003 microfarad.

' The resistor 264 is connected to the cathode of a diode 27 8, and the resistor 268 is connected to the 'cathode of a diode 280. The anodes of the diodes 278and 231i are grounded. i

The` primary winding 256 ofthe transformer 251i has a center tap connected* to a grounded capacitor 232 and to aresistor 284. The capacitor 282 may, for example, have a capacitance ofi microfarad. The resistor 284 may, for example, have ak resistance of 25 ohms, and it is connected to the positive terminal of the source 214.

The transistorized converter circuit 25d is a known type of push-pull transistor oscillator, and the oscillator produces an output signal across an output winding 236 of the transformer 25S. This output signal is passed through a" resistor 28S to a full-wave bridge rectifier.

290. The resistor Zrnay,l sistance `of 2;7 kilo-ohms.V Y

The resulting direct current output voltage from" the converter circuit 25) appears across the full-wave bridge rectifier 29). One terminal of the bridge rectifier, 291i for example, have a rekis connected through a `resistor 224 to a pair of seriesconnccted resistors 292 and 293. The resistor `292 has a value, yfor example, of 27 kiloohms and is grounded. The resistor 293 is connected to the point 232, and it has a value, for example, of 91 kilo-ohms. The resistor 294 may, for example, have a resistance of 6.8 kilo-ohms. The resistors 292 and 293 are shunted by a capacitor 296 which may, for example, Shave a capacity of .015

microfarad. The other terminal of the bridge' rectier 290 is connected to the lead 264 and to a resistor 298. The resistor 298 may, for example, have a resistance of 10 kilo-ohms, and it is, connected to the cathode of a diode 300.

The source 214 is .shunted by a series connected re- Y sister `302 and capacitor`i304. The resistor 3612 may yhave a resistance, for example, of 10 ohms, and` the.

capacitor'304 may have a capacity of, yfor example, 100 microfarads. The junction of the resistor 3tl2 and the capacitor 304 is connected to the collector of an NPN transistor 3l6 through a resistor 3dS. The resistor 3tlg may, for example, have `a resistance of 2 kilo-ohms.

,'Theemitter of the transistor 306 is grounded, and the collector is connected through a resistor'31tl to the base of a second NPN transistor 312. The resistor 310 may,

for example, have a resistance of 2 kilo-ohms. The

emitterof the transistor 312is grounded, and the base of the transistor is connected to a grounded resistor 314. The latter resistor may, forexample, have a resistance of 3.3 kilo-ohms.

The collector of the transistor 312 is connected through a pair of diodes 316 and 318 and through a resistor ii 321i to the base lof the transistor 306. The resistor 32 may, for example, have a resistanceof 1() kilo-ohms. The base of the transistor 306 is connected toa grounded resistor 322 which is shunted by a diode 324 and by a capacitor 326. The resistor 322 may have a resistance, for example of 2 kilo-ohms and the capacitor 326 may, for example, have a capacitance of .(lllmicrofarad.

The base of the transistor 3ti6lis also connected to the anode of the diode 30d, to the anode of a diode 301, and to the anode of a Zener diode 328. The cathode of the diode 3611 is connected to the resistor 220. The cathode of the Zener diode328 is connected through a resistor 330 to the junction of the resistors '292 and 293. The resistor 33@ may, for. example, have a resistance of 20 kilo-ohms. The transistor 396 is normally conductive, and the transistor 312 is normally nonconductive. f

In the event of ia mislire yand resulting short circuit condition iin the circuitof FIGURE 2, ythe misire protection circuit 13 of FIGURE 3 'functions to disable the silicon 126 and 128 reduces @the anode voltage of the siliconv controlled rectiiiers 1=12and 114 to a point at which they become non-conductive. n v

Under nonmal operation of the solid state switching circuit of FIGURE 2, the point 232 and the ranode of the Y sil-icon controlled rectifier 210 connected thereto, :are established at -a direct current voltage of the order of l volts by the converter 25d of FIGURE 3. This voltage is developed by the conve-rter in the manner described above, and it appears acnoss the rectilierbridge 290.

As described above, the presence yof a short circuit current in the lead 212 causes a pulse of voltage to appear across the winding l2212 of the magnetic core inductance device 210. This voltage causes the silicon controlled nectitier 210 to become conductive, which suddenly causes the anode thereof to be established -at ground potential.

The resulting negative-going pulse passes through the capacitor 230 to the lead 2014 to be applied to the terminal A of the circuit of FIGURE 2, and this latter pulse passes through the diodes 126 and 128 to the anodes of the silicon controlled rectiliers'1`12 and 114. At the sarne time, and in `a manner to be described, the lining signals are neirnoved lirorn the igate electnodes 'off the silicon controlled reotiers -112 and 11'4.

The negative-going pulse referred to above also passes through the voltage divider resistors 234 and 236`to control the current through the control winding 240 of the magnetic 'amplifier 146 of FIGURE 2. This control serves to remove the ring signal 'from the silicon convtrolled'rectifiers 120 and'122 'of FIGURE. 2.

23th inresponse tothe above-described short circuitucondition, 'the cathode of the diode 300` swings negative -to render the diode conductive. The conductivity of the diode 300 causes thev base of Ithe transistor 306 to swing negative causing that transistor to become non-conductive. When the transistor 306 rswings Ato its non-conductive state, the transistor 312`is caused to bzecornev conductive.

When the transistor 306 is vestablishedin its conductive condition, the resulting collector current of that 'transistor causes the potential of the lead 206, and of the terminal C of FIGURE 2, .to be drawn down to yground potential,

so that the diodes and 192 inthe circuit of'FIGURE ,theZener diodes i178 fand `184.

2 becomeconductive. The conductivity of the diodes ottime vsilicon controlled-rectiiers 120 and 122 through .Therefore,.theabove-mentioned short circuit condi-tion in the circuit of FIGURE 2 causes a negative Isignalto be Iapplied to the tenminals A and C by the misire pro tectiongoircuit 18 of; FIGURE 3. `This negative signal causes .the silicon controlled yrectii-ers r120 and 122 to he Vdeactivated ,in the above-described manner tov de-energize the system. 'i A At the tenmination ofi-the pulse across the Winding 222 of the magnetic `core-induction device 210, the capacitor 296 recharges tltroughthe Vresistors 293 Iand 292 and the potential at the point 232 increases exponentially in the positive direction. This increase continues until the threshold of the Zener ydiode 323i is exceeded, and the transistor 306 is yagain rendered conductive. The transistor 312 is then vdriven back to its non-conductive condition, and this causes the positive voltage to be introduced through the diodes 316 land 3h13` to the -base of the transistor 306.

Tne resultof the above-described faction is that a positive pulse is introduced through the Zener biode 328` to cause the transistor 306 to become conductive and the resistor 3G16 is heldin itsv conductive state by the positive voltage introduced to its base electrode from the collector of the non-conductive transistor 312 through the' diodes 316 and318. The transistor 306 remains conductive until another negative pulse is introduced to its` base electrode through the diode 300".

The 'action described in the preceding paragraphs enables the diodes 190 and 192 to be rendered non-conductive by the resulting positive voltage 'at the terminal C after a predetermined time interval, so that the tiring signals Ican automatically again be applied to the gate electrodes of the silicon controlled rectifers 120' and 122v tore-establish nonrnal operation .in the system.

` modifications las tall Within the scope ofv the invention.

What is claimed is:

l p l. In a system for producing an alternating current.l output `and which includes: gener-ating means for producing a pair of oppositely phased gate signals, fand first and second main switching devices coupled to said genf erating rneans to .be sequentially actuated by said oppositelyy phased gate signals in a succession of recurrent operating cycles, the combination of: a transformer inoludingla primary Winding having first `land second sides respectively yconnected to said rst and second main switching devices, said primary Winding further having a `center tap and rst and second intermediate taps disposed `lon opposite sides of said center tap, said transformer turther including a secondary Winding across which an v alternating current secondary "volt-age is produced, poten- Itial supplying means :connected to said center tap on said winding for introducing an exciting potenltial thereto, circuit means coupled to said transformer .for deriving a regulating signal representative orf the amplitude of said alternating current secondary Voltage,

.rst andsecond auxiliary switching devices connected respectively to said first and second main switching devices :and to said first land second intermediate vtaps on sai-,d primary IWinding, and a control circuit coupled to said circuitzmeans and to said -iirst and second `auxiliary switching devices tor enabling said first and second auxiliary lswitching'devi'ces to be sequentially conductive at f ll) points in successive cycles .of said alternating curren-t voltage las determined bynsaid regulating signal efr`iectively 'to lconnect said first and second main switching devices` to said iirstand second intermediate taps.

2. In a system tor producing an alternating current outputand IWhich includes:` ygenerating means for-producing apair` ofYopposi-telyl phased gate signals, and first and.secondlinainy switchingdevces coupledto said ygenerating means to be sequentially actuated by said oppositely phasedgate signalsfin succession oi recurrent operating cycles, the lcombination of: a transformer including a primary winding having first and second sides respectively connected to ksaid iirst and second` main switching-devices, said primary Winding further having a centertap and iir'st Iand second intermediate taps symmetrically disposed on opposite .sides of said center tap,

f said transformer .further'including asecondary Winding across whichan alternating current secondary voltage is produced, potential supplying means connected to said center tap on said primary Winding for introducing an exciting potential thereto, said transformer further including a control winding for producing an alternating current control signal related to the amplitude of said alternating current secondary voltage, circuit means coupled to said control Winding and including rectifying means for producing a direct current regulating signal having an amplitude related to the amplitude of said alternating current secondary voltage, first and second auxiliary switching devices connected respectively to said lirst and second main switching devices and to said first and second intermediate taps on said primary Winding selectively to connect said rst and second main switching devices to said intermediate taps, and a control circuit coupled to said circuit means and to said iirst and secon-d auxiliary switching devices 4for enabling said auxiliary switching devices to be sequentially conductive at points in successive half cycles of said alternating current secondaryy voltage as determined by the amplitude of said direct current regulating signal.

3. In a system for producing an alternating ycurrent output and IWhich includesi generating `means for producing a pair of oppositely phased gate signals, and iirst and second controlled solid state devices coupled to said generating means to be sequentially rendered conductive by said oppositely phased gate signals in a succession of recurrent cycles, the combination of: a transformer including a primary Winding having first and second sides respectively connected to said iirst and second controlled devices, said primary Winding further having a center tap and iirst and second intermediate taps symmetrically l. disposed on opposite sides of said center tap, said transof said alternating current secondary voltage; third and fourth controlled solid state devices connected respectively toy said first and'second controlled devices and to said first and second intermediate taps on said primary Winding selectively to connect said first and second controlled devices to said intermediate taps, and a control circuit coupled to said circuit means and to said third and fourth controlled devices for enabling said third and fourth controlled devices to be sequentially conductive at points in successive half cycles of said alternating current secondary voltage as determined by the amplitude of said direct current regulating signal.

4. T'hecombination delined in claim 3 yand lin which said control circuit `includes a magnetic amplifier for effectively simplifying the direct current regulating signal from said circuit means and for applying respective components of the amplieddirect current signal :to said third and `fourth controlled solid state devices.

,5. The combination defined in claim 1 and in which said potential supplying means includes a protective circuit responsive to a current flow in said primary Winding in excess of a predetermined threshold for disabling said irst and second main switching devices so as to interrupt i the current flow in said primary Winding.

6. The combination dened in claim 3 and in which said potential supplying means includesa protective circuit responsive to a current flow in said primary Winding in excess of a predetermined threshold for rendering said irst Yand second controlledv solid statefdevices non-con ductive, said protective circuit including: means interposed between the source of directicurrent exciting potential and said center tap on said primary Winding and responsive to a current iow in said primary Winding in excess of an established threshold for producing a trigger pulse, and means coupled to said rst and second controlled devices and, responsive to said trigger pulsef'for rendering said iirst and second controlled devices nonconductive.

References Cited in thele of this patent Vi UNITED STATES PATENTS Younkin.4 Nov. 8, 1960 Van Emden Nov. 211, 1961 OTHER REFERENCES Lilienstein. 

1. IN A SYSTEM FOR PRODUCING AN ALTERNATING CURRENT OUTPUT AND WHICH INCLUDES: GENERATING MEANS FOR PRODUCING A PAIR OF OPPOSITELY PHASED GATE SIGNALS, AND FIRST AND SECOND MAIN SWITCHING DEVICES COUPLED TO SAID GENERATING MEANS TO BE SEQUENTIALLY ACTUATED BY SAID OPPOSITELY PHASED GATE SIGNALS IN A SUCCESSION OF RECURRENT OPERATING CYCLES, THE COMBINATION OF: A TRANSFORMER INCLUDING A PRIMARY WINDING HAVING FIRST AND SECOND SIDES RESPECTIVELY CONNECTED TO SAID FIRST AND SECOND MAIN SWITCHING DEVICES, SAID PRIMARY WINDING FURTHER HAVING A CENTER TAP AND FIRST AND SECOND INTERMEDIATE TAP DISPOSED ON OPPOSITE SIDES OF SAID CENTER TAP, SAID TRANSFORMER FURTHER INCLUDING A SECONDARY WINDING ACROSS WHICH AN ALTERNATING CURRENT SECONDARY VOLTAGE IS PRODUCED, POTENTIAL SUPPLYING MEANS CONNECTED TO SAID CENTER TAP ON SAID PRIMARY WINDING FOR INTRODUCING AN EXCITING POTENTIAL THERETO, CIRCUIT MEANS COUPLED TO SAID TRANSFORMER FOR DERIVING A REGULATING SIGNAL REPRESENTATIVE OF SAID AMPLITUDE OF SAID ALTERNATING CURRENT SECONDARY VOLTAGE, FIRST AND SECOND AUXILIARY SWITCHING DEVICES CONNECTED RESPECTIVELY TO SAID FIRST AND SECOND MAIN SWITCHING DEVICES AND TO SAID FIRST AND SECOND INTERMEDIATE TAPS ON SAID PRIMARY WINDING, AND A CONTROL CIRCUIT COUPLED TO 